1. Field of the Invention
The present invention pertains to the art of microwave cooking appliances and, more particularly, to a microwave cooking appliance including a heating element mounted within a cooking chamber such that impedance characteristics of the heating element substantially match impedance characteristic of the cooking chamber in order to reduce the occurrence of high and low electric fields.
2. Discussion of the Prior Art
The art of cooking is currently undergoing substantial change. It is no longer the norm to have a family member home all day with time to cook and prepare meals. Today, more and more consumers must rush home from work to prepare meals for themselves or for their families. In today""s fast paced society, time is of the essence. The luxury of spending time in preparing a meal is becoming less and less affordable. As such, consumers demand an oven that will cook a meal in less time than conventional ovens, without sacrificing the quality of the prepared food. In order to meet these demands, manufacturers are combining conventional cooking systems with the rapid cook advantages of microwave cooking systems.
Cooking appliances utilizing a directed microwave energy field to cook a food item have existed for some time. In such a cooking appliance, food is heated by directing standing microwaves into a cooking chamber where the microwave energy is directed upon the food item to be cooked. As the microwaves are reflected within the chamber, they impinge upon the food item, causing the food item to undergo a cooking process. The nature of the standing waves often results in localized areas of high and low energy fields which, coupled with other factors, cause the food to cook unevenly. This is especially true in larger ovens where the size of the cooking chamber requires a more uniform energy distribution in order to properly cook the food. In addition to confronting the design challenges related to incorporating microwave cooking into larger ovens, combining conventional cooking elements with the microwave systems requires specific considerations relating to the internal geometry of the cooking chamber.
For example, introducing conventional electrical heating elements into a microwave oven chamber will impact the impedance characteristics of the chamber. Not only do the microwaves reflect from the chamber walls, but the microwaves would also reflect from the heating elements themselves. Accordingly, the number of modes, and thus hot and cold spots resulting from high and low energy fields, would increase dramatically.
Several methods have been proposed in the prior art to address problems with these methods ranging from providing shields for the heating elements in the form of panels, or locating the heating elements below a food support or the like. Each of these arrangements adds to the cost, and can actually detract from the overall efficiency of the system. Other manufacturers choose to ignore the problem entirely in simply incorporating heating elements within the cooking chamber. In any event, despite the prior art arrangements, there exists a need for a microwave cooking appliance employing an electric heating element designed to be incorporated into a cooking chamber wherein the impedance characteristics of the element substantially matches the impedance characteristics of the cooking chamber, thus reducing the effects of moding during a cooking process.
The present invention is directed to a microwave cooking appliance including an electrical heating element mounted within a cooking chamber. Specifically, the electrical heating element is preferably constituted by sheathed, resistive electrical element defining a broil element having impedance characteristics which, when properly arranged, substantially match the impedance characteristics of the cooking chamber. More specifically, the microwave cooking appliance includes a cooking chamber having interior and exterior upper surfaces, a toroidal-shaped waveguide including a bottom surface having an interior diameter and an exterior diameter, a waveguide input extending from the waveguide, a magnetron arranged on the waveguide input, and a sheathed electric heating element arranged on the interior upper surface of the cooking chamber, wherein the waveguide and electrical heating element includes complementary configurations designed to enhance the overall cooking performance of the appliance.
In a preferred embodiment, the sheathed resistive electric heating element includes an outer portion extending about and preferably suspended from the interior upper surface of the cooking cavity, an inner portion extending about an inner diameter of the bottom surface of the waveguide, and a cross-over portion electrically interconnecting the outer and inner portions. In accordance with the invention, the heating element is sized such that the element provide a sufficient amount of heat output necessary to perform a variety of cooking operations. In addition, the element is shaped and located so as to tune the impedance characteristics of the element. In this manner the heating element acts in a manner similar to an RF antenna, directing RF energy within the cooking chamber to areas of low electric field concentrations.